Poring over CT scans of the Papua New Guinea species Trigonopterus oblongus, Riedel was surprised to find that its legs appeared to be screwed into its body. The top segment of each leg, the femur, is attached to a small part called the trochanter. In turn, the trochanter attaches the leg to the insect's body, by screwing into a part called the coxa, which is similar to a hip.

Inside the coxa and on the outer surface of the trochanter, Riedel found ridges just like those on screws and nuts. The beetles could twist their front legs through 90 degrees and their middle and hind legs through 130 degrees.

Flexible weevils

Wondering if the design was unique to T. oblongus, Riedel looked at 15 other weevil species from different families. They all had the same screw-and-nut mechanism. "It's a safe bet that all weevils have it," he says.

The ability to twist its legs in their sockets might be particularly handy for weevils like T. oblongus, which lives on leaves and twigs and consequently has to splay its legs to find footholds. Not all weevils live like this, but the earliest ones probably did, explaining why the mechanism is ubiquitous in modern weevils.

It looks like the beetles are using the screw-and-nut mechanism in the opposite way to humans, says Chris Lyal of the Natural History Museum in London. Humans turn a screw to make it move along its length, but the weevil's muscles pull along the length of its leg to make it turn.

The weevils are another example of evolution coming up with the same solutions to problems as human engineers. Bacteria had continuously rotating wheels long before we did, in the form of spinning "tails" called flagella. It seems the weevils beat us to threaded screws and nuts. "Insects are fabulous," Lyal notes.

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